Ethylene-based polymer materials are generally known in the art. For example, polymers and blends of polymers have typically been made from a linear low density polyethylene (LLDPE) prepared using Ziegler-Natta and/or metallocene catalyst in a gas phase process. Films made from conventional Ziegler-Natta catalyzed LLDPE's (ZN-LLDPE) are known to have favorable physical properties such as stiffness, lifting ability and tear resistance, but poor impact resistance. Films made from metallocene catalyzed LLDPE (m-LLDPE) are known to have superior impact resistance and suitable stiffness, but often suffer from drawbacks, such as low tear strength, in both the machine and transverse film directions, compared to films prepared with ZN LLDPE. Thus, the film industry has sought metallocene catalyzed film resins that exhibit favorable stiffness and tear resistance similar to, or better than, those prepared using ZN LLDPE resins, while retaining the superior impact resistance of films prepared using m-LLDPE reins. Specifically, the film industry wants films having a stiffness exceeding 200 MPa and both MD Elmendorf Tear and Dart Drop values equal to or exceeding 20 g/micron.
The film industry is still in search of methods and compositions that overcome these shortcomings and provide improved physical properties, improved processability, and an improved balance of properties.
U.S. Pat. No. 6,242,545 describes a process for the polymerization of monomers utilizing hafnium transition metal metallocene-type catalyst compound. The patent also describes the catalyst compound, which comprises at least one cyclopentadienyl ligand including at least one linear or isoalkyl substituent of at least three carbon atoms.
U.S. Pat. Nos. 6,248,845 and 6,528,597 describe single reactor processes for the polymerization of monomers utilizing a bulky ligand hafnium transition metal metallocene-type catalyst compounds. These patents also describe an ethylene polymer composition produced by using bulky ligand hafnium metallocene-type catalysts.
U.S. Pat. No. 6,956,088 describes metallocene-catalyzed polyethylenes having relatively broad composition distribution and relatively broad molecular weight distribution. Specifically, U.S. Pat. No. 6,956,088 discloses thin (about 0.75 mil, 19 micron) blown films made from ethylene polymers made using a bis(n-propylcyclopentadienyl)hafnium dichloride and methylalumoxane that are reported to have a superior balance of stiffness, tear resistance, and impact resistance. However, this superior balance of properties can only be obtained under selected film fabrication conditions requiring extensive draws and high stretch rates. The metallocene-catalyzed polyethylenes of U.S. Pat. No. 6,956,088 lose their superior balance of film properties when made under typical draws and stretch rates used to make the majority of commercial films. In addition, these polyethylene films lose the superior balance of film properties as the gauge of the film is increased to be greater than about 0.75 mil (19 micron).
U.S. Pat. Nos. 6,936,675 and 7,172,816 describe polyethylene films produced from a polymer obtained using a hafnium-based metallocene catalyst. Methods for manufacturing the films are also described. These films do not have a balance of softness (lower 1% Secant Modulus), greater lifting ability (Tensile at Yield), and lower Ultimate Strain/Ultimate Stress ratios.
U.S. Patent Application Publication No. 2008/0038533 (specifically Examples 46, 47 and 48) discloses films made from polyethylene made from catalyst systems disclosed in U.S. Pat. No. 6,956,088. These films do not have a balance of softness (lower 1% Secant Modulus), greater lifting ability (Tensile at Yield), and lower Ultimate Strain/Ultimate Stress ratios.
U.S. Pat. Nos. 7,179,876 and 7,157,531 disclose films made from ethylene polymers made using a bis(n-propylcyclopentadienyl)hafnium metallocene and methylalumoxane. These films do not have a balance of softness (lower 1% Secant Modulus), greater lifting ability (Tensile at Yield), and lower Ultimate Strain/Ultimate Stress ratios.
There is a need to produce a LLDPE resin giving films having the maximum tear and impact resistance shown in the table with sufficient stiffness for processing through conventional film handling equipment. These target properties are: stiffness (as measured by MD 1% Secant Modulus) exceeding 200 MPa (32 K psi) and both MD Elmendorf Tear and Dart Drop values of at least 20 g/micron (500 g/mil) with balanced impact and tear resistance (Dart Drop/MD Elmendorf Tear ratio of about one).
This invention provides polyethylene and films thereof having improved physical properties, improved processability, and improved balance of properties.
Likewise, trimethylaluminum (TMA) has been used in some polymerizations as a scavenger, although some gas phase polymerizations prefer no scavenger such as TMA (see U.S. Pat. No. 6,956,088, column 5, lines 18-25, citing WO 96/08520).
Methylalumoxane (MAO) is often used as an activator with metallocene catalyst compounds and one common method of making MAO is the hydrolysis of TMA. Such hydrolysis however tends to leave residual TMA in the MAO which can have negative effects on polymerization.
WO 2004/108775 discloses “[a]dditional components, such as scavengers, especially . . . alkylaluminum dialkoxide compounds and hydroxyl containing compounds, especially triphenylmethanol, and the reaction products of such hydroxyl containing compounds with alkylaluminum compounds, may be included in the catalyst composition of the invention if desired.”
Others have noted that an increase in amounts of AlMe3 in MAO can decrease catalytic activity in a number of systems, such as ring-opening polymerization of beta-lactones (see Organometallics, 1995, Vol. 14, pp. 3581-3583, footnote 5) and that trimethylaluminum does not appear to act as a co-catalyst (see Macromol. Chem. Phys., 1996, Vol. 197, pp. 1537-1544). Likewise, EP 1 650 231 A1 discloses that if a sterically hindered Lewis base is added to MAO, the TMA can become trapped and thus be prevented from interacting with the cationic species.
The reaction of triphenylmethanol and trimethylaluminum is disclosed in Harney D. W. et al., Aust. J. Chem., 1974, Vol. 27, pg. 1639.
Reddy et al. in Macromolecules, 1993, 26, 1180-1182 disclose that with increasing addition of free TMA to MAO both catalyst productivity and molecular weight decrease but that in homogeneous catalysts for ethylene polymerization, addition of TMA to MAO under specific conditions can lead to a dramatic increase in catalyst productivity and lifetime.
Other references of interest include US 2005/0282980; Busico, et al. Macromolecules 2009, 42, 1789-1791; and Imhoff, et al., Olefin Polymerization, Chapter 12, pages 177-191 (published by American Chemical Society, 1999).
This invention also provides a process utilizing TMA in combination with MAO to achieve enhanced polymerizations as well as enhanced product properties, such as enhanced tensile performance of polyethylene films. The processes disclosed herein offer the previously unknown ability to alter polymer microstructure and physical properties by manipulating the amount of TMA, or unknown species (defined below) in a MAO/TMA solution. In particular, the processes disclosed herein offer the possibility to influence the (intra- and/or intermolecular) comonomer distribution and/or the molecular weight distribution in a copolymer by adjusting the amount of TMA in a catalyst system.
The disclosed processes also offer on-line control in a continuous process of polymer microstructure and physical properties by means of controlling on-line the amount of TMA and/or an unknown species present in the MAO solution as described hereinbelow.